Looking at the pictures on their web page the wires connecting to the sensor board go black, red, green for pins 1, 2 and 3. So I would assume that 1 is 0v, 2 is +5v and 3 is the signal wire. But I don't have one, so don't blame me if you blow up your Arduino...

One of the pictures (http://www.robotshop.com/Images/xbig/en/dfrobot-digital-vibration-sensor-1-B.jpg) shows it connected to a "Roboduino I/O Expander. (http://www.robotshop.com/dfrobot-i-o-expansion-board-4.html)"

So based on those two pictures it appears

Pin 1: Black : GNDPin 2: Red : DPin 3: Green : +5V

Those colors really don't make sense (neither does an undocumented sensor), but that's the best I can tell without seeing an actual board.